Fourth generation (4G) cellular networks employing newer radio access technology (RAT) systems that implement the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and LTE Advanced (LTE-A) standards are rapidly being developed and deployed within the United States and abroad. LTE-A brings with it the aggregation of multiple component carriers (CCs) to enable this wireless communications standard to meet the communication bandwidth requirements of multi-carrier systems that cumulatively achieve data rates not possible by predecessor LTE versions.
Many different types of 4G LTE-compliant communication devices have been introduced into the consumer electronic device marketplace. Modern 4G communication devices are capable of communicating within heterogeneous telecommunication networks that employ a diverse mixture of 4G, third generation (3G), and second generation (2G) RAT systems. Accordingly, these multi-mode communication devices are configured with radio frequency (RF) circuitry that allows the devices to readily transition between LTE and LTE-A network cells, as well as to roam amongst and transition between various 3G and 2G legacy network cells. By way of example, 3G legacy RATs may include Universal Mobile Telecommunications System (UMTS)/High Speed Packet Access (HSPA) and Code Division Multiple Access (CDMA) 2000/1× Evolution-Data Only (1×EV-DO) systems, whereas 2G legacy RATs can include Global System for Mobile Communications (GSM)/Enhanced Data Rate for GSM Evolution (EDGE) systems.
Additionally, 4G multi-mode communication devices can be configured to perform different voice calling functions using either the packet-based voice-over LTE (VoLTE) communication protocol or a circuit-switched 3G or 2G legacy communication protocol. The choice of which communication protocol the mobile device is to utilize primarily depends on what RATs are available to the device at its present network location. For instance, when a multi-mode communication device is residing within the coverage area of an LTE or an LTE-A network cell, the device can be configured to initiate a voice call using the VoLTE communications protocol. However, in another scenario, when the same device is residing within the coverage area of a 3G or 2G legacy network cell and LTE network access is otherwise unavailable, the multi-mode device may be configured to initiate a voice call using a predecessor circuit-switched communications protocol.
Generally, multi-mode communication devices are configured to prefer attachment to 4G LTE networks offering relatively high data-rate throughput, as opposed to, attaching to 3G or 2G legacy networks offering substantially lower data-rate throughput. However, for voice calling, high data-rate throughput may not be necessary as voice calls are typically low-bandwidth, periodic communications. Accordingly, in certain scenarios, it may be more efficient for a mobile device to establish a voice call using a network cell with the most reliable radio coverage at the device's present location, as opposed to establishing the voice call using a network cell that employs a 4G RAT (offering a higher data-rate throughput) but has less reliable radio coverage at the device's current location. This scenario may exist when a multi-mode communication device is residing within, or is moving into, a cell edge region of an LTE network cell, where one or more overarching 3G or 2G network cell(s) can offer the multi-mode communication device more reliable voice calling service.
Moreover, some network operators with 4G LTE and LTE-A system deployments support VoLTE calling without enforcing VoLTE call continuity, such as in situations where a mobile device that is engaged in a VoLTE call roams outside the coverage area of its serving LTE network cell. In this scenario, a corresponding VoLTE call will be dropped at a time when LTE communications service is lost by the roaming device.
Accordingly, in the absence of strict call continuity enforcement by network operators, there is a need for an improved device mobility solution to prevent dropped calls when VoLTE-compliant mobile communication devices roam into regions with sparsely deployed, intermittently deployed, or no deployed 4G LTE network cells.